The present invention pertains to optical analysis of a cornea. In particular, the present invention pertains to a method and apparatus for mapping a corneal contour and thickness profile.
Accurate measurement of a corneal contour and thickness profile is important for photo-refractive surgery. These data are important to ensure surgical accuracy and documentation. For laser-in-situ-keratomileusis (xe2x80x9cLASIKxe2x80x9d) in particular, such data are essential to ensure proper surgical decision making and to ensure safety.
It is well known in the art that corneal topography provides curvature information relating to the anterior corneal surface. However, in practice, this information alone does not provide enough information for a surgeon to determine whether a LASIK surgical procedure can be performed safely and effectively.
It is also well known in the art that Scheimpflug slit lamp photography can be used to investigate the cornea and anterior chamber of an eye. U.S. Pat. Nos. 5,512,965 and 5,512,966 of inventor Richard K. Snook (xe2x80x9cthe Snook patentsxe2x80x9d) disclose a method and apparatus for measuring a corneal contour and thickness profile based on digitized slit lamp photography. In accordance with the Snook patents, a slit of light from an incandescent lamp is scanned over a cornea being examined to record a sequence of slit light images of the cornea with a CCD camera. The images are stored in a digital format and are analyzed to reconstruct the corneal contour and thickness profile. Further, a device referred to as an ORBSCAN II(copyright) slit-scan based, corneal and anterior segment topography system has been developed and manufactured by Orbtek, Inc. of Salt Lake City, Utah (now a part of Bausch and Lomb) based upon the disclosure of the Snook patents. The ORBSCAN II system device provides corneal information such as, for example, an elevation map, a curvature map, a power map, and a thickness (pachymetry) map.
One limitation of the method and apparatus disclosed in the Snook patents is the use of conventional slit lamp based technology. One limitation of conventional slit lamp based technology is that the slit lamp beam projects a focused beam with a rather short confocal length. As a result, the slit lamp image is not always sharp across the corneal surface. A further limitation of the slit lamp based technology is that only one slit image can be taken in each frame of a picture, and some forty (40) frames of pictures are required to achieve the required resolution. As a result, the sampling time is as long as a few seconds and patient eye movement becomes an issue. A yet further limitation of the slit lamp based technology is that a viewing angle along a slit lamp image varies from point to point. As a result, an algorithm used to reconstruct the corneal contour is rather complicated.
As a result, a need exists in the art for a method and apparatus for mapping a corneal contour and thickness profile that overcome the above-identified limitations of the slit lamp based technology.
Embodiments of the present invention advantageously provide method and apparatus for mapping a corneal contour and thickness profile that overcome the above-identified limitations of the slit lamp based technology. In accordance with one aspect of the present invention, an apparatus for mapping a corneal contour and thickness profile of a patient""s eye comprises: (a) a source which projects a set of beams of radiation onto a cornea of the patient""s eye; (b) a rotator which rotates the source; (c) a camera which acquires images of radiation scattered by the cornea; and (d) a controller that analyzes the images to construct the corneal contour and thickness profile.
In particular, in accordance with a preferred embodiment of the first aspect of the present invention, a set of beams of radiation is projected onto a corneal surface at an angle with respect to a predetermined axis (xe2x80x9can instrument axisxe2x80x9d). In operation, the instrument axis is substantially aligned with a visual axis of an eye, and the set of beams is rotated about the instrument axis. A camera (for example, a CCD camera) is disposed to view the cornea along the instrument axis. Since corneal stroma scatter the radiation more strongly than does the aqueous humor of the anterior chamber of the eye, traces of the rotating set of beams form a set of rings in images obtained by the camera. The outer and inner edges of the rings correspond to intersections of the set of beams with anterior and posterior surfaces of the cornea, respectively. Then, in accordance with the present invention, a direct triangulation algorithm is used to determine spatial positions of data points along the outer edges of the rings. These spatial positions are then used to reconstruct the anterior surface profile of the cornea. Next, using the anterior surface profile of the cornea, a ray tracing triangulation algorithm is used to determine spatial positions of data points along the inner edges of the rings. Next, these later spatial positions are used to reconstruct the posterior surface profile of the cornea. Finally, spatial differences between the posterior and anterior surface profiles of the cornea are used to generate the thickness profile. Advantageously, in comparison with the prior art technology, embodiments of the present invention provide: (a) a sharper beam image; (b) more data points for each frame of a camera image; and (c) a simpler algorithm for mapping the corneal contour and thickness profile.
In accordance with a second aspect of the present invention, an apparatus for mapping a corneal contour and thickness profile of a patient""s eye comprises: (a) a source which projects a beam of radiation onto a cornea of the patient""s eye; (b) a rotator which rotates the source; (c) a displacement mechanism which displaces the source; (d) a camera which acquires images of radiation scattered by the cornea at various displacements of the source; and (e) a controller which analyzes the images to construct the corneal contour and thickness profile.
In particular, in accordance with a preferred embodiment of the second aspect of the present invention, a single beam of non-coherent radiation is projected onto a corneal surface at an angle with respect to a predetermined axis (xe2x80x9can instrument axisxe2x80x9d). In operation, the instrument axis is substantially aligned with a visual axis of an eye, and the beam is rotated about the instrument axis. A camera (for example, a CCD camera) is disposed to view the cornea along the instrument axis using near confocal configuration. The beam is displaced slightly for each rotation and the camera acquires a series of images of rings which are analyzed as discussed above. In accordance with this embodiment, when the beam is displaced, an optical focusing system can also be displaced to maintain near confocal imaging.